1. Field of the Invention
This invention relates to a plastic filtration system for removing contaminants before product formation and more specifically to a diverter valve for directing the flow of viscous materials between alternate filters.
2. Description of the Prior Art
For a variety of manufacturing processes involving use of process fluids, such as "hot melts" and fluid monomer and polymer feed stocks, for example, nylon and polyesters, it is desirable to operate on a continuous flow basis. Such processes typically embody extrusion, injection, blow molding, coating and spraying techniques, for example, in the manufacture of synthetic textile fibers, plastic tubing, plastic sheets and films and protective or insulating coatings for electrical wires.
In such processes, it is common practice to include a filter unit in the flow stream to effect removal of impurities which might result in an imperfect product or cause clogging of downstream equipment such as spinnerettes or extrusion dies. As is obvious, filters must be cleaned or replaced periodically. Thus, to permit uninterrupted operation two parallel filter units (each of which may consist of more than one filter element) are provided together with changeover or diverter valves for alternatively diverting flow from one filter unit to the other so that one unit is in service while the other is being cleaned or replaced. It is usually desirable in such processes that filter changeover be accomplished without any significant reduction in process fluid flow or pressure to downstream equipment and without introducing air to the flow stream.
A major problem in successfully developing a system of the type above-referred to is to provide diverter valves which will operate reliably at the pressures (approximately 5000 PSI) and at the high temperatures (approximately 600 degrees Fahrenheit or more) within the highly corrosive environment provided by the materials being processed.
For the most part, the prior art diverter valves have used a rotary plug valve or a variation thereof to control the flow of the material being processed. Such valves comprise a circular or conical plug which fits within a mating opening provided in the valve body with the plug containing flow ports requiring critical matching to obtain the desired flow. Such rotary plug valves, when subjected to the pressures and temperatures required, have a tendency to freeze or seize up. If critical matching does not occur or a leak does occur, there is no possibility for the surface between the body and plug to be forced together to cure the defect. Some examples of such rotary plug prior art valves are found in U.S. Pat. Nos. 3,033,256, 3,059,276, 3,193,877, 3,243,849, 3,455,357, 3,480,706, 3,503,096, 3,679,060, 3,935,108, and 3,940,222.
In some instances in the prior art, slide plates have been utilized in order to divert the flow of the material being processed. However, such slide plates, as provided in the prior art, typically require non-metallic resilient seals to control or prevent leakage. Such seals again will not withstand high temperatures and pressures for long periods of time, thus providing a valve which is not reliable except for extremely short periods or in a different environment. Some examples of devices utilizing slide plates of various types within a plastic filtration system are shown by the following U.S. Pat. Nos. 2,661,497, 2,763,308, 2,786,504, 3,007,199, 3,059,276, 3,112,525, 3,145,746 and 3,503,096. In addition thereto, various slider valves have been known for a long period of time for diverting fluid flow for various other purposes. Some patents directed to such structures issued in the United States are as follows: U.S. Pat. Nos. 444,438, 994,544, 1,855,913, 1,930,827, 3,089,512, 3,103,233, and in addition thereto attention is called to British Pat. No. 845,187, the complete specification of which was published Aug. 17, 1960.
It has been suggested that a spool valve may be utilized to overcome some of the prior art problems. See U.S. Pat. No. 3,833,121. Although such a valve does overcome some of the problems, there is nonetheless a remaining problem evident in the spool valve, as well as in many of the prior art valves of the rotary, plug or slide plate types. That is, the construction in the flow areas through which the material being processed must move define isolated areas which are subject to stagnation. That is, the material being processed will move into these isolated areas and remain there. Depending upon the type of material involved, degradation will occur with extremely deleterious effects on the material being processed. For example, if polyvinyl chloride is being processed, the stagnant material will form hydrochloric acid and carbon and will then commence to increase in size (grow) until it extends into the flow of the material being processed and breaks off and contaminates the entire process. If other types of materials, such as polyethylene are being processed, the material will become cross-linked or carbonized in the stagnant area and will cause gels to form.
It is thus an extremely important feature to provide a diverter valve for controlling the flow between the filters of the material being processed that will not generate or provide any isolated areas into which the material being processed may flow and there stagnate. Such must be provided while still providing a valve which will operate reliably over long periods of time at the high pressures and high temperatures encountered with those materials and will not seize up or leak and which may be readily operated when desired while still providing the ability to easily and readily service the same should such be required.
Other prior art U.S. Patents relating generally to the art and known to applicant are U.S. Pat. Nos. 3,501,806, 3,145,746, 3,353,211, 3,488,806, 3,480,706, 3,727,767, 2,095,064, 3,396,845, 2,709,451, 3,025,873, 2,307,585, 3,135,284, 3,643,692, 3,746,481, 3,817,668, 2,039,858, 2,186,266, 2,709,451 and 3,154,485.